Superlative photoelectrochemical properties of 3D MgCr-LDH nanoparticles influencing towards photoinduced water splitting reactions

In the present work, we report the synthesis of single system three-dimensional (3D) open porous structure of MgCr-LDH nanoparticles in a substrate-free path by using one-step formamide assisted hydrothermal reaction followed by visible light irradiation for significant photoelectrochemical (PEC) properties that manifest towards photocatalytic H2 and O2 production. The as-prepared nanostructured materials were characterized by various physico-chemical characterization techniques. Moreover, this unique synthetic approach produces 3D open porous network structure of MgCr-LDH nanoparticles, which were formed by stacking of numerous 2D nanosheets, for effective light harvestation, easy electronic channelization and unveil superlative PEC properties, including high current density (6.9 mA/cm2), small Tafel slope of 82 mV/decade, smallest arc of the Nyquist plot (59.1 Ω cm−2) and photostability of 6000 s for boosting water splitting activity. In addition, such perfectly self-stacked 2D nanosheets in 3D MgCr-LDH possess more surface active defect sites as enriched 50% oxygen vacancy resulting a good contact surface within the structure for effective light absorption along with easy electron and hole separation, which facilitates the adsorption of protons and intermediate for water oxidation. Additionally, the Cr3+ as dopant pull up the electrons from water oxidation intermediates, thereby displaying superior photocatalytic H2 and O2 production activity of 1315 μmol/h and 579 μmol/h, respectively. Therefore, the open 3D morphological aspects of MgCr-LDH nanoparticles with porous network structure and high surface area possess more surface defect sites for electron channelization and identified as distinct novel features of this kind of materials for triggering significant PEC properties, along with robustly enhance the photocatalytic water splitting performances.

and oxygen vacancy related defect sites for effective electronic transportation resulting magnificent PEC properties towards photoinduced water splitting reactions 73, 74 . This resourceful practice certifies single-step synthesis of colloidal MgCr-LDH/NS and thanks to the oxygen vacancies on the MgCr-LDH/NS which mostly provided active sites for further nucleation and crystallization process. The growth process of the 3D MgCr-LDH/NP structures could be described as shown in Fig. 1; a significant and time-saving methodology has been adopted to deliver the significant structural transformation of exfoliated MgCr-LDH/NS to hierarchal 3D structure of MgCr-LDH/NP matrix. Firstly, the well-controlled growth of MgCr-LDH/NS from MgCr-LDH/ PS was accomplished by the use of hydrolyzing agent HCHO 75 , together with the OH¯ by using coprecipitation method and dispersion through sonication process 76 . Mostly, hydrolysis of HCHO liberates solvation energy 77 , which prepared the mixed solution of Mg(NO 3 ) 2 ·6H 2 O and Cr(NO 3 ) 3 ·9H 2 O, more alkaline and triggers nucleation and growth of MgCr-LDH/NS owing to restricted access to nutrients in confined area. At some point in the reaction process, HCHO acts as ligand binding the Mg 2+ and Cr 3+ cations to produce metal complexes in aqueous medium through H-bonding; and causes the complex configuration of polyoxometalate cluster. Thirdly, at mild hydrothermal process of 80 °C, these metal cluster complex shape into 1D sequence using hydrolysis reaction; and chain segments are united to form supramolecular units. Under such instance, the nucleation and growth process of LDH in the successive reaction with OH − and HCHO could hinder so causes the formation of MgCr-LDH/NS. With continuous heating at 80 °C for 12 h, the incremental thickness of the interconnected NS crystallizes into fully-fledged NS. However, after a visible light irradiation of about 30 min, the exfoliated NS entangled and folded to self-stack and cluster shaped into 3D NP. However, the transition state of the MgCr-LDH/NS happened through an unusual route, with advancement of porous 3D MgCr-LDH medium. The morphological alteration started from 2D NS to 3D NP formed by aggregation, self-assembly, and Ostwald process 72 . Further decomposition of HCHO releases formate together with slow liberation of NH 3, CO 2 , H 2 and H 2 O in a restrained gap 50 , but ensure for the porosity and floppiness in the material. In the interim, several H 2 O molecules also penetrate into the interlayer 49, 50 . The main reaction steps of MgCr-LDH/NS to MgCr-LDH/NP transformation are given below.  Step-1 Step-3 Step-2 Step-4 Step-5 Mg , forming Mg(OH) 2 that offer the nucleation site for Cr 3+ ions to precipitate (Ksp = 1.6 × 10 -30 ) as Cr(OH) 3 .Though, Mg 2+ and Cr 3+ ions coordinated with CHO − ions and generated [Mg (CHO) x ] 2−x and [Cr(CHO)y] 3−y intermediates, the H + and OH − ions neutralize to fix the solution pH at 7. In these circumstances, nucleation and growth process of LDH by reacting with OH − and NO 3 − could be arrested, leading to the creation of MgCr-LDH/NS. The MgCr-LDH/NP would be generated by self-assembly of freshly created MgCr-LDH/NS (Fig. 2a) on the previously formed layers (Fig. 2b). As discussed, the layered 2D MgCr-LDH/NS interconnected to create 3D NS consisting of 2D NS with an open structure; besides, these kind of morphological aspects furnish an enormous amount of available surface sites, which manifest enrich photo/electroactive sites for the water redox reaction, and open space for ion pooling for escalating the kinetics of diffusion barrior within the electrode/electrolyte interface 78 .
Following the FESEM analysis, the structural aspects of the 3D MgCr-LDH/NP, could be well-recognized vide transmission electron microscopy (TEM) and high resolution-TEM (HR-TEM) analyses. TEM images of MgCr-LDH/NS ( Supplementary Fig. S1a,b), and MgCr-LDH/NP (Fig. 2c) elucidate the effect of HCHO induced mild hydrothermal treatment and visible light irradiation on structure and morphologies of materials. Figure 2c exemplified the distinct and fluffy nature of the characteristic 3D MgCr-LDH materials. Further the TEM image also illustrated the consistency of dense and thin 2D NS ( Supplementary Fig. S1a,b), in typical 3D MgCr-LDH/ NP (Fig. 2c) 79 . The free and exposed 2D NS surface ease out catalyst reactions and triggers the photocatalytic water splitting activities of binary MgCr-LDHs 80 . Furthermore, the obscure part appeared in Fig. 2c was owing to the dense stacking, and distortation of the NS and these properties could be identified in graphene and analogus materials 81 . The high resolution-transmission electron microscopy (HR-TEM) images of MgCr-LDH/NP (Fig. 2d)   Structural, surface area and valence state features of MgCr-LDHs. The solid state crystallographic planes of MgCr-LDHs based samples were characterized through powder XRD (PXRD) pattern and the entire diffraction pattern could be resembled into hexagonal crystal phase with space group R3m of rhombohedral symmetry of hydrotalcite like materials (Fig. 3). The diffraction pattern of MgCr-LDH/PS (Fig. 3a), consisting of three main peaks at 2Ɵ = 13.9°, 30.8°, and 55.0° could be ascribed to the phase reflection of (003) plane, edge plane of (006), along with (110) plane 38,82 , which are approximately matching with the JCPDS file No. 01-089-0461. The (012) and (110) edge planes in XRD pattern of LDH are considered as the main exposed planes and match up to the cationic and anionic distances within the layered structure. The peak index of the (110) reflection plane was found approximately at 2Ɵ = 55.0° and evidence the retaining of the LDH layered structure 38 . The higher shifting of the (003) and (006) planes together with the lower shifting of (110) basal reflection plane suggested a change in the unit cell parameter and decline in the periodicity of basal planes. This is related to the H 2 O content from the interlayer LDH galleries. The interlayer-spacings (d) were calculated by the use of Braggs law, nλ = 2d sin (Ɵ), where n = 1, λ = wavelength of the target, and Ɵ = incidence angle. The d-spacing value of MgCr-LDH/PS related to (110) plane was calculated to be 1.66 Ȧ, which is of typical characteristic of NO 3 − intercalated LDH materials.
Alternatively, the PXRD pattern of MgCr-LDH/NS (Fig. 3a), clearly disclosed broad and symmetrical basal reflections at lower 2Ɵ = 34.4°, corresponding to the (012) basal planes with little spike type of asymmetrical reflections at higher 2Ɵ = 56.9°, assigned to the (110) planes owing to the HCHO induced exfoliation process, which are partially matching with the JCPDS file No. 01-089-0461. The reduced intensity and significant higher shifting of the (003) plane at 2Ɵ = 18.3°, and missing of the intense (006) planes compared to MgCr-LDH/ PS, indicated with the interlayer height differences, and change in basal spacings and stacking disorder due to the variation in water contents and formation of discrete nanosheets under the influence of HCHO induced exfoliation 76,77 . The d-spacing value of MgCr-LDH/NS related to the (110) plane was calculated to be 1.6169 Ȧ.
In contrast, the MgCr-LDH/NP (Fig. 3b), exhibits sharp and broad reflection of the main exposed planes of (003), (012) and (110) at 2Ɵ = 12.9°, 34.7° and 60.6°, respectively (JCPDS file No. 01-089-0461). The relatively shifting of the broad reflections peaks of MgCr-LDH/NP to higher 2Ɵ angle in comparison to MgCr-LDH/ NS is quite indicative of the decrease in the interlayer distance, which is an indicative of the cross-assembling of the nanosheets and corresponding evolution of the bundles of nanoparticles in 3D open structure. This consequences are further verified by the decrease in interlayer distance of 1.5267 Ȧ relative to the (110) basal planes of MgCr-LDH/NP. Furthermore, the slight and less intense growth of the (006) basal reflection planes at 2Ɵ = 23° demonstrate a reduction in periodicity of basal reflection plane owing to the cross-association of nanosheets to form bundles of nanoparticles. This implies that the crystal sizes are reduced in both lattice parameter a (a = 2d(110)) and c (c = 3d(003)) directions, indicative of self-stacking thickness of LDH nanosheets in 3D assembly of nanoparticles. These results showed that there were no other impurity phases detected in the PXRD pattern of MgCr-LDH based nanostructure materials during the structural variation from MgCr-LDH/ PS to MgCr-LDH/NP through MgCr-LDH/NS. The variations in crystallographic information of MgCr-LDH based samples are included in Table S1.
The Fourier transform infrared (FT-IR) spectroscopy ( Supplementary Fig. S3) also exploits the alteration of molecular units during the formation of MgCr-LDH/NP. In the case of MgCr-LDH/NS, the strong and broad   84 . In addition; an additional absorption band at 1642 cm −1 corresponds to the deformation of H 2 O molecules 85 . The insignificant band at 652 cm −1 collectively with the band at 1450 cm −1 are related with the overlapped NO 3 − bending mode of vibration with the unwanted carbonate groups perhaps contaminated from the CO 2 gas of air 86 . Mostly absorption bands beneath 800 cm −1 can be accredited to M-O bending and stretching mode of vibrations. In the interim, the FT-IR spectra of MgCr-LDH/NP signifies extremely broad shoulder peak of -OH functional group at 3400 cm −1 and the missing peak of -OH group approximately at 3000 cm −1 , demonstrating that the coordinated -OH groups with the metal cation of hydroxide layers exist in different phase and possess defect sites. Furthermore, the decrease of the peak intensity at 1450 cm −1 and 652 cm −1 signifies that NO 3 − and CO 3 2− anions are completely eliminated after a hydrothermal treatment and light irradiation 85 . Similarly, the absorption bands beneath 800 cm −1 in MgCr-LDH/NP can be accredited to the M-O bending and stretching mode of vibrations. As the LDH layered structure is stabilized by the electrostatic interactions among the hydroxide layer and intercalated anions; so the elimination of NO 3 − anion in MgCr-LDH/NP by stumbling of formamide assisted hydrothermal and light treated exfoliation usher to diminish their interactions, which in succession causes delamination of cationic layers and further self-assembling of the nanosheets and aggregated into prosper like a nanoparticles in 3D structure as confirmed from the TEM analyses of the material structure (Fig. 2c).
The as-synthesized MgCr-LDH based materials were further characterized by the N 2 adsorption-desorption isotherms (relative pressure (P/Po) vs. volume of N 2 adsorbed) in order to study the surface area, the average pore volume and the mesoporosity nature of the samples, which could have remarkable effect on the electrochemical properties and photocatalytic performance of the materials. All of these materials displayed type-IV isotherm with H1 hysteresis loop and shows mesoporous characteristics. The BET surface area of MgCr-LDH/ PS, MgCr-LDH/NS and MgCr-LDH/NP were found to be 45, 96 and 115 m 2 /g respectively (Fig. 4a,b). The pore diameter of the as-synthesized MgCr-LDH/NP material are found to be 5.57, 7.71, 12.13 nm and supports the mesoporous nature as similar to its nanosheets and pristine materials as depicted in Fig. 4c. The increased volume of mesoporosity in 3D MgCr-LDH/NP represents the secondary pores, which arises due to the swelling behavior of OH − groups induced by HCHO together with removal of gaseous ions during the constant hydrothermal    Table 1, outlined the surface area, pore diameter and pore volume of the MgCr-LDH based materials. Hence, the wide pore volume of MgCr-LDH/NP indicates the mesoporosity characteristic with high surface area that promoted more reactive sites available at the surface of catalyst which could enhances the rate of water splitting reactions. The X-ray photoelectron spectra (XPS) elucidate the surface elemental composition and surface states of binary MgCr-LDH based catalysts (Fig. 5). The presence of Mg, Cr, O and C elements were noticeable on the XPS surface survey spectra ( Supplementary Fig. S4). Figure 5 represents the deconvoluted Gaussian-fitted XPS spectra of Mg 2p, Mg 1s, Cr 2p, O 1s and C 1s in the modified MgCr-LDH/NS and MgCr-LDH/NP based materials. In an illustration, Fig. 4a shows the Mg 2p XPS spectra of MgCr-LDH/NS and MgCr-LDH/NP. In MgCr-LDH/NS, for the Mg 2p 3/2 spectrum (Fig. 5a), peak located at 49.8 eV reveals the occupancy of Mg(OH) 2 and corresponded to the main Mg 2+ cationic states in the material 47 . Moreover, peak fitted Mg 2p 3/2 spectrum of MgCr-LDH/NP shows the Mg 2+ -cationic states after the structural transformation into nanoparticles (Fig. 5a). However, the corresponding Mg 2p 3/2 peaks of MgCr-LDH/NP were blue-shifted to higher binding energy 50.1 eV (difference in energy shifting ∼ 0.3 eV). The fitting XPS spectrum of Mg 2p in MgCr-LDH/NP reveals the existence of bivalent Mg 2+ in material. Figure 5b, showed the appropriate binding energy of Mg 1s peak of MgCr-LDH/NS at 1302.9 eV 49 , whereas the Mg 1 s peak of MgCr-LDH/NP was identified at 1303.1 eV, which noted the absolute continuation of Mg 2+ states in MgCr-LDH. Figure 5c showed the XPS spectrum of Cr 2p of MgCr-LDH/NS. The energy level fitted Cr 2p 3/2 and Cr 2p 1/2 peaks were appeared at 576.4 and 586.5 eV in the, respectively 49 . The binding energy of Cr 2p peak at 576.4 eV denoted the creation of Cr-O bond 70 . Similarly, the core-level Cr 2p XPS spectrum of MgCr-LDH/NP, could be fitted into two spin-orbit doublets, which corresponded to the peaks of Cr 2p 1/2 and Cr 2p 3/2 for the presence of Cr 3+ cation 74 . The binding energy of Cr 2p at 577.2 and 586.5 eV was accredited to the Cr 2p 3/2 and Cr 2p 1/2 states, which verified the trivalent nature of Cr ions. The binding energy of Cr 2p at 577.2 eV proposed the generation of Cr-OH bond. These results suggested that the metal cations associated with MgCr-LDH/NP preserved the unusual valence state after the hydrothermal and light treatment. As illustrated in Fig. 5c, the Cr 2p peaks in MgCr-LDH/NP slightly shifted towards higher binding energy in comparison to the Cr 2p peaks in MgCr-LDH/NS. These results might be attributed to the successful introduction of Cr 3+ with empty electron orbitals, which adjusts the electronic structure of the catalyst. Figure 5d displays the O 1s XPS spectra of MgCr-LDH/NS and MgCr-LDH/NP. The high resolution O 1s XPS spectrum of MgCr-LDH/NS could be deconvoluted into three peaks at 530.8, 531.3 and 531.6 eV, which are assigned for lattice oxygen linked with Mg and Cr metal, surface hydroxyl bonded to metal centers and oxygen vacancies or under-coordinated lattice oxygen vacancies 49 . Moreover, in comparison to the O1s spectrum of MgCr-LDH/NS, the approximate peaks identified in MgCr-LDH/NP includes 529.7, 530.5, 531.3, and 531.6 eV, which is associated for water molecules, lattice oxygen, surface -OH group, and oxygen vacancies, respectively 73 . Moreover, the more prominent oxygen vacancies peak in MgCr-LDH/NP signifies the subsistence of oxygen vacancies related to defects type owing to existence of delaminated MgCr-LDH during the hydrothermal process and further light irradiation causes aggregation of the nanosheets to produce MgCr-LDH/NP containing oxygen vacancies sites. Figure 5d, shows that the hydrothermal treatment enhances the intensity of the M-OH bond, and formation of oxygen vacancies on assembly of nanosheets in MgCr-LDH/NP becomes more favorable at an optimal light exposure time of 30 min. The percentages of Ov as determined from the fitted peak area of O 1s spectra are 25% and 50% for MgCr-LDH/ NS, and MgCr-LDH/NP samples. Further hydrothermal treatment with light exposure causes appearance of new peaks attributed to the formation of adsorbed water peaks at 529.7 eV 73 . In addition, the positively shifted Cr 2p 3/2 peak of MgCr-LDH/NP (~ 577.2 eV), demonstrated the decreases in electron density around Cr and electron clouds are inclined towards the Mg(OH) 2 surface owing to the formation of oxygen vacancies. Additionally, the C 1s XPS spectrum of MgCr-LDH/NP (Fig. 5e) revealed the existence of C 1 s main peak with high binding energy at 289 eV corresponded to O−C=O linkage. The other binding energy peaks at 287.9, 286.7, and 284.5 eV corresponded to C-O-C, C-OH, C-C linkage, respectively 17 . All of these characteristic features substantiate that hydrothermal treatment of formamide treated bulk MgCr-LDH could led to the removal of gaseous products like NO 2 from the interlayer of LDH and causes structural twist towards nanosheets with oxygen vacancies and further exposure under visible light resulted with self-aggregation and removal of other gaseous products like CO 2 , H 2 , and H 2 O in a sintered confinement, thereby leading to nanoparticles like MgCr-LDH/NP containing exfoliated self-stacked nanosheets with enriched oxygen vacancies. Hence togetherness of hydrothermal and visible light treatment has dramatic effect on structural twist from bulk MgCr-LDH to nanoparticles through nanosheets for significant PEC properties and photoinduced water splitting reactions.
PEC properties studies of the MgCr-LDH material. The magnificent PEC photocurrent properties of the MgCr-LDH/PS and the corresponding MgCr-LDH nanosheets and hierarchical 3D MgCr-LDH/NP structure were investigated using LSV studies as obtained under dark and visible light illumination in order to legacy Table 1. BET Surface area, pore diameter and pore volume of the MgCr-LDH/PS, MgCr-LDH/NS and MgCr-LDH/NP samples.

Samples
Surface area (m 2 /g) Pore diameter (nm) Pore volume (cm 3 /g)     54 . At the meantime, no noticeable input from the dark current scan could detect for all electrodes in the entire investigated potential window. The onset potential is determined by the junction point of the light current density and dark current density in the j-V curve 87 . The generation of the photocurrent at the onset potential of a semiconductor photoelectrode explores their catalytic tendency towards redox activities. The shift in the onset potential of the nanostructured material reveals the structural transition with enhanced photoelectrochemical redox reaction activities. Particularly, low onset potential reveals the minimum loss of energy during the electrochemical redox reactions. The onset potentials in Fig. 5a are recorded at 0.9 V while Fig. 5b shows the onset potential at 0.2 V vs. RHE. The unusual big difference of onset potentials of approximately 700 mV was detected among these electrodes, which might be due to the three-dimensional structure of MgCr-LDH/NP containing dispersed nanoparticles, reducing the recombination of the charges and promotes the charge transfer. Hence, the onset potential of MgCr-LDH/NP is greatly decreased through the structural transition from bulk to nanosheets, and then to 3D morphological features of nanoparticles, which is indicative of the amalgamation of oxygen vacancies related defect sites for easy charge tunneling and fast separations of excitonic charge pairs for enhanced photocatalytic water oxidation performances. The Tafel slope is mostly utilized to authenticate the superior OER properties of various binary LDH, which is considered as the rate determining step in the water splitting process; and is deliberate by below equation 88 .
where ƞ, a, b, and j correspond to the overpotential, constant, Tafel slope, and current density. The Tafel slopes were rationalized from the LSV polarization curves at scan rate of 10 mV s −1 by plotting potential (V) vs. log (j) (RHE), where the LSV curves were counted at a particular region starting from the onset potential where current density starts to increasing. The linear fitting of the top portion of the Tafel plot gives rise to the Tafel slope. The calculated Tafel slopes are 239, 192, and 82 mV/decade for MgCr-LDH/PS, MgCr-LDH/NS and MgCr-LDH/NP, respectively (Fig. 6c). It was found that morphological variation from bulk to nanosheets and further nanoparticles like assembly decreases the Tafel slopes and the smallest slope was tenable for MgCr-LDH/NP, confirming the highest current density and faster kinetics towards water splitting reactions. Normally, lower overpotential and smaller Tafel slopes constituted better catalytic water splitting performance and well-recognition to the fast electron-hole transfer and separation process owing to extraneous and uncovered active sites in 3D MgCr-LDH/  7. In the Nyquist plot of MgCr-LDH based electrodes, R1 is noted as the series resistance of the circuit, which is related to the charge-transfer resistance at the interface of Pt counter electrode/electrolyte at high frequency region of the semicircle. The R2 is noted as the charge-transfer resistance (Rct) at the interface of working electrode (MgCr-LDH)/electrolyte in the mid frequency region of the semicircle. The R3 is noted as the charge-carrier-transfer resistance in the Helmholtz double layer. The CP1, CP2 and Z W correspond to the chemical capacitance and Warburg impedance, respectively. Normally, the electrochemical model circuit suggested that the minor semicircle portion is related to the charge transfer resistance (Rct) and the major straight line is relevant to the mass transfer resistance (Rm) at low frequency 89 . The as-obtained fitted values of Rct for MgCr-LDH/PS, MgCr-LDH/NS, and MgCr-LDH/NP photoelectrodes were found to be 129.18, 80.80 and 59.17 Ω cm −2 , respectively. The as-obtained MgCr-LDH/ NP displays smallest Rct value among the three types of MgCr-LDH-based photoelectrode, which indicate the efficient dynamics of carrier charge separation and rapid surface redox kinetics, occurred on the MgCr-LDH/ NP photoelectrode and electrolyte interface. Moreover, the stability of the photoelectrode is highly necessary to secure high PEC efficiency of the materials. The stability of the MgCr-LDH/NP photoanode samples was tested performing chronoamperometric J-T curve measurements by applying a constant potential of 0.5 V to overcome the Ohmic losses in the electrolyte and metal contacts under visible light exposure for 6000 s ( Supplementary  Fig. S5). Interestingly, rational photocurrent stability preservation over a suitable period was exemplified for the MgCr-LDH/NP nanostructure photoelectrode.
On the other hand, the frequency dependent Bode phase angle plot of MgCr-LDH electrodes are shown in Fig. 8, and they are used to measure the electron lifetime in the nanostructured materials. Normally, the highest peak intensity of the Bode phase angle curve stipulates the rate of the charge transfer at the electrode interface. The Mott-Schottky plots were acquired for MgCr-LDH/PS and nanostructure MgCr-LDH photoelectrode samples indicating the reversed sigmoid plots resembling to n-type semiconductors (Fig. 9). A flat band potential (V fb ) of an electrode could be calculated by following Mott-Schottky equation 47 , where ε is the dielectric constant, N is the the charge carrier density, C is the space charge layers capacitance, Va is the applied potential, e is the electron charge, and ε 0 is the permittivity of vacuum. The estimated V fb value recedes in the potential edge of CB (E CB ) of n-type semiconductors vs. RHE. Furthermore, the carrier charges density (Nd) found from the Mott-Schottky plots was used to estimate the alteration in carrier charge concentration. The theoretical equation calculating Nd of semiconductor is as follows.
Importantly, the flat band potential of MgCr-LDH/NP indicated decrease in band bending and higher slope assigned to the increased in carrier density, which is attributed to the defect-sites allowed to the charge transfer process among the electrode and electrolyte. Hence, the significant charge transfer rate in MgCr-LDH/NP photoelectrode is a synergistic result of 3D flower like structure containing 2D nanosheets and oxygen related  Figure 10a, reveals that as the structural transformation increases from the bulk phase to nanosheets and gradually increases towards nanoparticles, the H 2 and O 2 production shows a volcanic trend. The enhanced water splitting activity of binary MgCr-LDH/NP might be owing to the distinctive structural features (3D nanoparticles contented with self-stacked 2D nanosheets) and the synergistic effects among the dispersion of Mg, and Cr atoms as found from the TEM results. Figure 10a, shows the maximum hydrogen production of MgCr-LDH/NP reaches to 1315 μmol/h, which was 1.  54 . This is also reflected in the XPS spectra and impedance plots of the magnificent PEC properties, and the formation of nanoparticles structure is more conducive to H 2 production because of the special structure of the layered 2D nanosheets inside the 3D nanoparticles offers added active phases, which amplify the excitonic separation process, so facilitates quick redox reaction. The existence of inconsistent oxidation states in the binary LDH (Mg2þ/1s and Cr2þ), due to the inclusion of Cr 3+ in the framework, charge transfer, conductivity and electron capture hastily followed to facilitate the H 2 production. Furthermore, the fabricated photocatalysts were also examined towards O 2 evolution reaction (E 0 O 2 /H 2 O = + 1.23 V vs. RHE), under 250 W visible light emitting Hg-lamp for a period of 1 h 90 . Figure 10b showed that the MgCr-LDH/NP displayed the highest O 2 production activity of 579 μmol/h followed by the MgCr-LDH/NS of 356 μmol/h and MgCr-LDH/PS of 254 μmol/h. The enhanced production capacity of MgCr-LDH/NP was due to the similar reason as explained for H 2 production, i.e., owing to the presence of rich defect site related to oxygen vacancies trap out more photoexcited electrons, that would be available over Moreover, the 3D structures with high surface area intimately allocate the 2D active nanosheets, which could render additional active sites, and assist excitonic charge transportation due to porosity by the release of gaseous products in the 3D nanoparticles architecture. The addition of Cr 3+ cations is supposed to be potentially redox active sites in the MgCr-LDH OER catalyst. In order to measure the photostability of MgCr-LDH/NP catalyst during the H 2 and O 2 evolution, a cyclic H 2 and O 2 evolution experiment was carried out using 10% CH 3 OH and AgNO 3 aqueous solution (Fig. 10c,d). Each cycle was 125 min, and total of 4th cycles was performed during the experiment. In the 3rd and 4th cycle, the H 2 and O 2 production gradually decreases due to the consumption of sacrificial reagents. The H 2 and O 2 evolution showed that the MgCr-LDH/NP photocatalyst possess good catalytic stability. In addition, XRD patterns were executed on the MgCr-LDH/NP photocatalysts before and after the cycle of H 2 production, as shown in Supplementary Fig. S6. It was found that the XRD patterns of the catalyst before and after the H 2 production cycle did not change significantly, except a little reduction in peak intensity which may be due to loss in catalyst handling, surface blocking by the sacrificial reagents and may be corrosion of catalysts surface during the catalytic reaction. Similarly, the TEM image of the MgCr-LDH/NP after fourth runs of the H 2 evolution test reveals no significant changes in the phase and morphology (Supplementary Fig. S7). These features indicated that the MgCr-LDH/NP catalyst possess excellent water splitting activity.
Additionally, the H 2 production experiment of MgCr-LDH/NP was carried out under the presence of different sacrificial agent [10% lactic acid solution, 10% methanol, 10% triethanol amine (TEOA)] under similar experimental condition as shown in Fig. 11a. The sacrificial based water splitting reaction depends upon various factors such as the oxidation potential of the reagent, polarity, chain length, side-product formation, adsorption on catalyst surface, number of hydroxyl groups etc. Experiments showed that the highest H 2 production was with the 10% CH 3 OH aqueous solution. This is because of the easy electron donor in the reaction system, and more electrons are generated and transferred to the active part of the photocatalyst for H 2 generation reaction; further the reagent oxidized by photogenerated holes in the VB of LDHs to CO 2 . The mechanism detailed is as predicted in the following equations. www.nature.com/scientificreports/ Further, scavenger experiment was performed to trace out the active species responsible for water oxidation by using different sacrificial agents such as AgNO 3 , isopropyl alcohol (IPA), ethylenediamine tetraacetic acid (EDTA-2Na) as displayed in Fig. 11b. It was pragmatic that the O 2 formation activity is maximum in case of AgNO 3 , whereas on addition of IPA, and EDTA-2Na (hole scavenger), the reduction performance increases which indicates the active role of hole in the water oxidation process. Yet again, for quantifying the efficiency of the photocatalyst towards O 2 production, the apparent conversion efficiency, was measured to be for photocatalytic O 2 evolution by MgCr-LDH/NP system under visible light irradiation. Considering this results, the ·OH radical formation was experimented over different as-synthesized samples (MgCr-LDH/PS, MgCr-LDH/ NS, and MgCr-LDH/NP) and the result depicted the highest possible formation of ·OH radicals, signifying the most resolute photoluminescence (PL) peak of the terephthalic acid (TA)-OH complex over MgCr-LDH/NP as shown in Fig. 11c. The ·OH formation ability of the MgCr-LDH/NP could be regarded as the effective separation of excitonic pairs via appropriate amount of oxygen vacancies and Cr 3+ dopant for enhancing the kinetics of water oxidation leading to greater accumulation of highly oxidizable holes in the VB of the concerned material. Moreover, the calculated VB potential of MgCr-LDH/NP was approximately 2.0 eV vs. NHE, which is quite sufficient enough to generate ·OH radical (OH/·OH = 1.99 eV vs. NHE). Hence the formation of e − , h+ and ·OH radical is quite feasible over the surface of MgCr-LDH/NP for superior photocatalytic water splitting performances.
Insight into the possible photocatalytic mechanism of charges separation. Ultraviolet (UV)visible (Vis) diffuse reflectance spectra (DRS) and PL spectra were analyzed to explore the optical properties and electronic charge transfer path within the MgCr-LDH based photocatalyst 16,17,21 . The optical absorption properties of a photocatalyst/photoanode are an important phenomenon, which directly affect their photocatalytic performances 92 Ag 0 n → Ag n , www.nature.com/scientificreports/ of the exposed atomic sites of the nanolayers that minimized the electronic transfer distance and formation of oxygen vacancies as revealed from the XPS spectra, which allowed for dense concentration of electronic clouds over the nanosheets with enhanced conductivity for photoinduced catalytic performances. Intriguingly, MgCr-LDH/NP, owing to the dynamics in structure with more defect sites endorsed numerous lights to scattered inside the folded and aggregated nanosheets to strengthen the optical path. The most interesting findings of MgCr-LDH/NP are of red-shifted light absorption intensity in comparison to MgCr-LDH/NS and MgCr-LDH/PS, respectively. Moreover, the intense defect site in terms of oxygen vacancies in MgCr-LDH/NP, amplify the absorption of light intensity in the wider visible zone for enhanced photocatalytic performances. As displayed in Fig. 12a where α is the absorption coefficient, hν is the incident photon energy, A is a constant, E g is the band gap energy, respectively. The plot of (αhν) 2 as Kubelka-Munk function vs. hv as function of photon energy gives the band gap energy value of MgCr-LDH based samples by using linear plot ranges extrapolated to the hv axis intercept (Fig. 12b-d). LDHs appeared to have a multifaceted band structure, which could be ascribed to the multiple band gaps, notifying the occurrence of different types of electronic transitions within the material 16,17,21,45 . Similar structure was identified in MgCr-LDH, howbeit it displayed three optical bandgap related to three absorption bands and accounts for directly allowed transition as versified from Fig. 12a- In addition, the band gap alteration of the as synthesized MgCr-LDH/NP is influenced by the defect site specific to oxygen vacancies, which could enhances the light absorption intensity in the visible region for significant photocatalytic water splitting performances.
Photoluminescence (PL) spectral technique is a fundamental tool to analyze the transfer and separation efficacy of photoinduced excitonic charge pairs in various semiconductor photocatalytic materials 16,17,21,45 . When the molecule absorbs light energy, first it would become in the excited state. However, the electrons in excited state have a short lifespan. If they do not react in time, they would be dissipated in the form of fluorescence and heat and the utilization rate of visible light of the catalyst might be reduced. The faster is the quenching of molecules in excited state of electrons, then higher the steady-state fluorescence emission peak intensity of the molecule (Fig. 13). The weaker PL signal signifies the higher lifetime of photogenerated charge carriers in semiconductor photocatalyst. Herein, PL was used to investigate charge transfer behavior of structurally evolved MgCr-LDH based materials starting from the bulk phase to nanosheets and then nanoparticles at an excitation wavelength of 320 nm as shown in Fig. 13 45 . The main peak of MgCr-LDH/PS is centered at approximately λ = 374 to 410 nm, which is associated with the typical photoemission of MgCr-LDH, approximately close to the bandgap energy of 3.7 eV (E g 1) 49 . The emission peak at 400-410 nm in MgCr-LDH/PS is due to the vacancies in MgO 6 octahedron, which acts as recombination sites and used to trap holes. The emission peaks at 459 nm could be linked to the radiative recombination of surface trapped localized excitonic charge carriers. The large decrease in PL intensity for MgCr-LDH/NP indicated that the recombination of photogenerated exciton pairs is significantly quenched owing to the large density of formation of defect sites and oxygen vacancies after the structural evolution from bulk to nanosheets and then self-assembling of the nanosheets led to the formation of nanoparticles 91, 94 . This is related to the dynamic of charge transfer within the MgCr-LDH/NP matrix, which could be helpful to stimulate the PEC properties and corresponding water splitting reaction. The PL spectra of MgCr-LDH/NS and MgCr-LDH/NP also reveals three types of characteristic emission band comprising of vacancies exist in MgO 6 octahedron of the Mg(OH) 2 layers, localized surface defect, and oxygen vacancy. The localized defect state and oxygen vacancies in MgCr-LDH/NS arises owing to the presence of uncoordinated metal centers during the formation of nanosheets and triggers charge transfer inside the Mg(O) 6 octahedron and towards the Cr(O) 6 octahedron. However, the rich defects sites and oxygen vacancies peaks of MgCr-LDH/ NP was identified at 500 nm and 524 nm, respectively, which was due to the occupancy of the numerous folded nanosheets during the secondary growth period of nanoparticles structure to reduce their surface energy, and release of the strong stress, under exterior forces for instance electrostatic, van der Waals forces, and hydrogen bonds in which twisted nanosheets self-assembled into stable and irregular 3D nanostructures 91 . MgCr-LDH/PS displays the strongest PL peak signifying higher efficiency of excitonic recombination process. The most diminished PL peak of MgCr-LDH/NP at about 373-500 nm reveals the lower recombination rate of photoinduced excitonic pairs. Hence, the suppression in excitonic charge pairs in MgCr-LDH/NP is associated with electron and hole trapping sites, which increases the fate of electronic charge pairs for trigging superior water splitting performances. Generally, the smaller the impedance arc radius, the faster the charge carriers separation. The In summary, the MgCr-LDH/NP combination can not only use the internal oxygen vacancy and Cr 3+ dopant as barrior for the electron-hole recombination to accelerate the separation of carriers, but also build an effective electron transfer channel, accelerate electron transfer, and improve the charge trapping ability.
In general, photocurrent response is used to reveal the phenomenon of photogenerated electrons generated by photoexcitation of photocatalyst. As we all know, the higher the photocurrent response value, the higher the excitation rate of photo-induced exciton pairs, and minimize the electrons and holes recombination rate. The transient photocurrent responses of three working electrodes under visible light exposure are revealed in Fig. 6d. It could be identified that after structural transformation into MgCr-LDH/NP, the catalyst formed successfully constitutes a dense of nanosheets containing oxygen related defect sites, and the MgCr-LDH/NP working electrode shows a significant increase in photocurrent density. An internal interface is formed within the nanoparticles structure where oxygen vacancies and Cr 3+ involved in the multi electron process for effective trapping of the electrons separating out from the photogenerated holes for superior water splitting reactions. Moreover, the mesoporosity nature of the as-synthesized MgCr-LDH/NP materials offers high surface area plus more surface active sites for photoelectrochemical reactions to enhance the water splitting performance.
In order to further analyze the electron transfer within the catalysts, Mott-Schottky curves and UV-DRS plots were correlated to calculate the CB and VB edges, respectively. The flat band (E fb ) of n-type semiconductor is close to the conduction band 17 45 . The XPS and PL spectra also verify the presence of defect site and oxygen vacancies. Moreover, Cr 3+ cations present electronic arrangement (t 3 2g e 0 g ), which induces charge transfer, separation and electronic capture for facilitating the H 2 production. These features provide strong support that the upward shifting of energy level is related with the successful formation of nanoparticles (verified from TEM and FESEM results) with defect sites as oxygen vacancies and Cr 3+ as dopant for triggering excitonic separation.
With these valid discussions, the possible CB and VB position of MgCr-LDH/NP and the mechanism of water reduction and oxidation reaction over MgCr-LDH/NP were proposed in Fig. 14. With the visible light irradiation, semiconductors could absorbed photon energy equal to or greater than the band gap energy, and get excited to produce electrons and hole pairs. The photogenerated electrons transition from the VB position of MgCr-LDH/NP to the CB, and leaving behind holes in the VB. The electrons accumulated on the CB of MgCr-LDH/NP are easily trapped by the Ov center together with the Cr 3+ cations presents unique electronic   54 . The V O percentage of MgCr-LDH/NP was higher than MgCr-LDH/NS as verified from the peak area fitting in the XPS spectra. Moreover, the LSV curve is also in agreement of defect sites for high current density. Furthermore, water oxidation intermediates are more favorably adsorbed on oxygen vacancies with the help of doped Cr 3+ in pulling up their electrons. The corresponding Tafel slope is 82 mV/decade and these results confirm that the incorporation of Cr 3+ is the crucial factor in increasing the reaction kinetics of MgCr-LDH/NP. Cr 3+ as Lewis acid cations can modulate the ligand fields of the hydroxyl groups of LDH layers. In this way the electrons are concentrated in the CB and attracted towards V 0 of Cr 3+ cations and thereafter in the CB of MgCr-LDH/NP and holes intense at VB of MgCr-LDH/NP (+ 2.0 V vs. RHE) possess sufficient potential to produce ·OH radicals E Ѳ (·OH/OH-= + 1.99 eV vs. NHE) 17 Fig. 15. Hence, the entire MgCr-LDH/NP can be regarded as a high-efficiency PEC cell assembly connected in three electrode series. This is advantageous to the improvement of hydrogen evolution performance. In addition, compared with other variant of LDH-based photoelectrode, the MgCr-LDH/NP photoelectrode also reveals comparable PEC properties, as shown in Supplementary Table S4.

Conclusions
In summary, we have successfully designed defect-rich 3D nanoparticles-like MgCr-LDHs composed of 2D nanosheets by using a facile hydrothermal and light irradiation method, and taken advantage of these special 3D nanoparticles-like structures that provided added active sites, thereby behave as an effective photocatalysts by reducing the recombination of photo-induced e− and h+ pairs, for enhancing the water splitting activities. In addition, XPS and PL analyses shows the dominance of oxygen vacancies and defects site with special electronic configuration of Cr 3+ dopant (t 3 2g e 0 g ), and synergistically facilitates charge transfer, conductivity, electron capture and adsorption of water oxidation intermediates for facilitating the H 2 and O 2 production. Moreover, the MgCr-LDHs nanoparticles delivered interesting PEC properties with low Tafel slope values of 82 mV/decade for a current density of 6.9 mA/cm 2 , which is significant and these LDH might be used as a potent photoanode material for future PEC water splitting activities. Evidently, MgCr-LDH nanoparticles exhibited superior photocatalytic H 2 evolution activities of 1315 μmol/h, which was 1.8  Photocatalytic water splitting measurement studies. The catalytic competence of the as prepared MgCr-LDH samples were tested towards water splitting reaction under visible light exposure from 125 W Xe lamp (power density = 100 mW cm −2 ) attached to a quartz reactor fitted with Julabo based chiller and 1 M NaNO 2 as UV cut off filter to filter out visible light of λ ≥ 400 nm. The water splitting reaction was begin with the addition of 0.02 g of catalyst to 20 mL of 10 vol% CH 3 OH solution and other sacrificial agents then purged with N 2 gas for 15 min to remove any dissolved O 2 gas to make the environment inert prior to light exposure. Then the reaction suspension was stirred continuously for 1 h to avoid any catalyst settlement under the exposure of visible light. The evolved gas was collected using downward displacement of water and further detected by GC-17A and column packed with 5 Å molecular sieves, set with thermal conductivity detector (TCD). Similar experiment condition was implemented, for O 2 evolution, with 0.03 g of catalyst added to 30 mL of 10 vol% of AgNO 3 and other tested sacrificial agents. Apparent Conversion Efficiency (ACE) for H 2 evolution = The apparent conversion efficiency (ACE) of the MgCr-LDH/NF photocatalyst producing H 2 gas of 1315 μmol/h and O 2 gas of 579 μmol/h by using 125 W Hg lamp as the visible light source positioned 9 cm away from the photocatalytic reactor could be determined by using the below mentioned Eq. (9) where ΔH c = heat of combustion of hydrogen in kJ/mol, Stored chemical energy = (number of moles of hydrogen produced per second) × ΔH c kJ/mol = 0.3652 × 10 -6 mol/s × 285.8 × 10 3 J/mol = 0.1043 J/s or W. The calculated power density for 250 W Hg lamps as visible light source is approximately 100 mW cm −2 . Incident photon energy = power density of the incident visible light × (light exposed spherical surface area of the reaction container) = 100 mW cm -2 × π × r 2 (r = radius of the spherical surface = 1.5 cm) = 100 mW cm −2 × 3.141 × (1.5 cm) 2

Materials characterization techniques.
The phase purity of the as-prepared materials were characterized by XRD, Rigaku Miniflex powder diffractometer) with Cu Kα as radiation source (λ = 1.54 Å, 30 kV, 50 mA). The functional groups associated with the bending and stretching mode of vibration of the materials were specified by JASCO FT-IR-4600, using KBr reference. The exterior surface morphology and structural features of the materials were obtained by FESEM by using ZEISS Sigma 500 VP microscope. The internal structure and morphology of the material was explored under the TEM and HR-TEM analysis by using JEOL 2100. The XPS measurement was taken at an X-ray photoelectron spectrometer (ESCALAB 250XI) with X-ray source as nonmonochromatized Mg Kα and energy of 0.8 eV. The optical absorption measurements were recorded by JASCO-V-750 UV-Vis spectrophotometer. The PL emission spectra were recorded by applying excitation energy of 320 nm using JASCO-FP-8300 spectrophotometer. The surface area of the MgCr-LDH based samples were measured by N 2 adsorption-desorption Brunauere-Emmett-Teller (BET) measurements using NOVA Quantachrome TouchWin v1 0. 22. The pore size distribution and pore volume were obtained by applying the BJH model. PEC measurements of samples were carried out by potentiostat-galvanostat (IviumStat) terminal.